Preparation of alkyl lactates



Dec. 4, 1945. s. M. wElsBERG ETAL 2,390,140

I PREPARATION OF ALKYL LACTATES I Filed Feb. 17, 1944 Patented Dec. 4,1945 PREPARATION F ALKYL LACTATES Samuel M. Weisberg and Edwin G.Stimpson, Baltimore, Md., and Alfred L. Miller, State College, Pa.,assignors to Sealtest, Inc., Baltimore, Md., a corporation of MarylandApplication February 17, 1944, Serial No. 522,712

8 Claims.

This invention relates to the production of alkyl esters of lactic acid.It relates particularly to the production of water-soluble alkyllactates from alkali metal or alkaline earth metal lactates.

As disclosed in the Weisberg et al. application, Serial No. 468,184,filed December 8, 1942, watersoluble alkyl lactates may be produced byesterifying an aliphatic alcohol with an alkali metal or alkaline earthmetal lactate in the presence of an acid catalyst, at a pH value betweenabout 0.7 and 1.4 to obtain a substantially quantitative conversion ofthe lactate salt to the alkyl lactate.

The esterication reaction requires an extended period of time, forexample, one-half to six hours, to go to completion, after which theester is separated together with water and alcohol from the solidcomponents present in the reaction chamber by flash distillation, forexample, in a heated oil bath. The alcohol and water are separated,successively, by distillation and azeotropic distillation to produce asubstantially anhydrous ester.

The above generally described method is quite satisfactory with theexception that the process requires too much time, is not particularlywell adapted to continuous operation, and contamination of the solidcomponents and ester by, and loss of, the oil during ash distillationsometimes occur.

An object of the present invention is to provide a simplified andcontinuous method of producing alkyl esters of lactic acid in a minimumamount of time.

Another object of the invention is to provide a method of producingwater-soluble alkyl esters of lactic acid in which esterification isconducted continuously and in a relatively short period of time withimmediate separation of the ester and other volatiles from solidspresent during the reaction.

A further object of the invention is to provide a method ofsimultaneously esterifying an aliphatic alcohol and an alkali metal oralkaline earth metal lactate and separating the ester and other volatileliquidsfrom the solids present during the reaction.

Other objects of the present invention will become apparent from thefollowing description of 1typical methods embodying the presentinvenion.

In accordance with the present invention an aliphatic alcohol, forexample, methyl, ethyl, propyl, butyl or other higher alcohol is reactedwith an alkali metal or alkaline earth metal lactate in the presence ofa strong mineral acid under conditions of temperature such thatesteriiication proceeds to completion substantially instantaneously andthe ester and other volatiles are ash distilled from any solids presentor formed during the reaction.

In order to obtain a substantially quantitative yield of the ester, ithas been foundkdesirable to have a relatively large excess of thealcohol present during the reaction, for example, about three to fifteenparts of alcohol to one part of lactate salt (calculated on a basis) byweight so that the mass reaction effect is such as to force the reactionin the proper direction. Moreover, the reaction is conducted preferablyat a pH value between 0.7 and 1.4, inasmuch as it has been found thatthe reaction will not go to completion at a higher pH value, asdescribed more particularly in application Serial No. 468,184.

The proper conditions for the esterication reaction may be obtained bydrying the lactate salt substantially to a water-free condition, mixinga little aliphatic alcohol withthe salt to render it fluid andthereafter mixing therewith an excess of the alcohol. Sulfuric acid isadded to the mixture in suiicient quantity to adjust the pH value to 1.4or less. The pH value may be determined in any suitable way, eitherduring mixing or on the basis of analyses of samples of the lactate saltprior to mixing. Inasmuch as the lactate salt contains materials thatact as buffering agents, the amount of acid added may vary considerablyfrom time to time in order to maintain i the desired pH value. v

During the mixing operation `the mixture may be preheated eithercontinuously or in batches, to below the boiling point of the ester, andat about or slightly below the boiling point of the alcohol, forexample, to about F. when producing methyl lactate to conserve heatdemand in the ash chamber.

The preheated mixture is then fed continuously into a iiash chamberheated to above the boiling point of the mixed vapors of ester, alcoholand water. Upon entry into the chamber, tle mixture reacts almostinstantaneously and flashes into a vapor containing the ester, excessalcohol and water. The non-volatile or solid components, such as analkali metal sulfate containing proteinaceous material, are heated to ahigh temperature, thereby giving up the ester occluded V therein.

The solid components may be discharged and treated, as by calcining, toproduce a suitable chemical by--product such as, for example, sodiumsulfate, when sodium lactate is used as a primary reagent.

The volatile components may be condensed and then further treated inorder to separate the a1- cohol and the water from the ester. Thealcohol may be removed readily by distilltaion or fractionation inasmuchas the boiling points of the alcohol, water and esterare suiciently farapart that a substantially clean separation may be obtained.

The water may be separated from the watersoluble esters by distilling itas an azeotropic mixture by adding, prior to or during heating, asuitable organic agent which forms an azeotropic mixture with water,boiling at a temperature below the boiling point of the ester. Many ofthe water-insoluble alcohols, ethers, ketones, hydrocarbons and estersare suitable for this purpose. Among the alcohols found useful arebutanol, iso-butyl alcohol and primary iso-amyl alcohol. Among thesuitable hydrocarbons are benzine and toluene. Among the ethers that areuseful are di-butyl ether and di-iso-butyl ether. Among the ketones thatare useful are methyl iso-propyl ketone, methyl iso-butyl ketone andmethyl-n-amyl ketone. Alkyl halides may also be used, if desired.

For a better understanding of the present invention, reference may behad to the accompanying drawing in which:

Figure 1 is a flow sheet diagrammatically illustrating the method; and

Figure 2 is a view in vertical cross-section of a typical form of devicefor flash esterifying the mixture.

.Referring now to Figure l. the alcohol, alkali metal or alkaline earthmetal lactate and sulfuric acid may be stored in suitable storage tanksI0, II and I2, respectively, from which they may be deliveredcontinuously by means of proportioning pumps I3 to a mixing chamber I4wherein the alcohol, lactate salt and sulfuric acid are intimately mixedand the pH value of the mixture adjusted to between about 0.7 and 1.4.The mixture may be heated during its passage through the mixing chamberI4. if desired, and then may be delivered continuously into a flashesterier I5 which is maintained at a temperature well above the boilingpoint of the mixed vapors of the ester. alcohol and Water. As shown inFigure 2. the ash esterifier may consist of a cylinder I6 having ajacket I1 extending throughout the major portion of its length forreceiving superheated steam. The temperature of the steam and thetemperature in the cylinder will depend largely upon the boiling pointof the mixture of the ester and other volatile components and the heatrequired to raise the temperature of the mixture suddenly to its boilingpoint. For the esterication of methyl lactate, for example, steam shouldbe supplied to the jacket I1 at about 100 pounds pressure.

Within the cylinder I6 is mounted a screw conveyor I8 which is driven bymeans of an electric motor I 9 through suitable power transmissionmeans, such as the belt drive 2|! illustrated.

The conveyor I 8 is utilized for moving the solid components that arepresent in the mixture or formed during the esterication reaction fromone end of the cylinder I6 to the other so that it may be dischargedcontinuously from the cylinder. Preferably, the conveyor I8 is rotatedat about 200 revolutions per minute in`order to throwh the solidcomponents outwardly against the walls of the cylinder I6 to assure itbeing heated to a sufciently high temperature to give vup any esteroccluded therein. The cylinder is provided with a discharge outlet 2|which communicates with a pair of spaced apart gate valves 22 and 23forming a solids-receiving chamber between them through which the solidsmay be discharged without escape of any substantial amount of the vaportherethrough. As illustrated, the discharge outlet 2| is surrounded by asteam coil 24 which holds the solids-receiving chamber at such atemperature as to prevent the condensation of vapor therein.

The vapor may be discharged through another outlet 25 and delivered to acondenser, not shown.

After condensation, the mixture of ester, excess alcohol and water maybe heated to distill off the alcohol and the mixture of ester and watertreated to remove the water. If the alkyl lactate is water-insoluble,the water can be removed by decantation. Water-soluble esters can berendered anhydrous by distilling the water as an azeotropic mixture asdescribed above and as disclosed more particularly in the Weisberg etal. application, Serial No. 468,184.

A typical example of the process, indicating yields, is as follows:

Crude sodium lactate which has been substantially freed from water byheating is diluted with a small amount of methanol in order to render itsufficiently fluid to handle and is accurately metered into the mixingchamber I4. At the same time methanol is accurately metered into thechamber at such a rate that four parts by weight of methanol enter thechamber I4 to each part by weight of sodium lactate, calculated on abasis of sodium lactate. A third stream consisting of commercialsulfuric acid is also metered into the chamber at such a rate as toobtain a pH value of about 1.4 in the completely mixed material leavingthe chamber I4.

The rate of flow of the sulfuric acid is determined by means of acontinuously recording pH meter in the eiliuent from the mixing chamberI4. However, samples may be taken from the mixer from time to time, ifdesired, in order to determine and adjust the pH value.

The mixture is fed into the ash esterier I5 at a rate of about fourgallons of the mixture per hour. This feed rate produces about fourpounds of solid residue, chiefly sodium sulfate, and delivers asvolatile material, about three gallons of methanol, .4 gallon of waterand four pounds of methyl lactate per hour. The temperature of the mixedvapors discharged from the flash esterier is about F. or well below theboiling point of methyl lactate. Similarly, when flash esterifying ethyllactate the temperature of the mixed vapors is about 212 F.

From the preceding description of typical methods embodying the presentinvention, it will be clear that a continuous method of preparing alkyllactates has been provided whereby an increased rate of production isobtained. Moreover, less complicated apparatus is required forpracticing the method and control of the operation is reduced to minimumcomplexity. More economical production of the ester results, inasmuch asonly the chemicals required in the reaction are added. Thus, the use ofan alkali to 4neutralize the mixture of ester, alcohol and acid.

following esteriflcation, is unnecessary.

While it is preferable to use an alkali metal or alkaline earth metallactate as a source of lactic acid for esteriilcation with an alcohol,because of economies in cost of the alkyl lactate, it will be understoodthat substantially waterfree lactic acid may be ilash esteried withabout 3 to 20 parts of an alcohol in the presence of sufiicient sulfuricacid to give the mixture a pH value between 0.7 and 1.4. The lactic acidcan be of any grade and is concentrated to a substantially Water-freecondition. By substantially Water-free we mean less than 25% water.There are fewer solid impurities present in the mixture than when analkali salt is used, but the flash-esterication operation may be used toseparate the volatiles from any non-volatile impurities that may bepresent.

Also a mixture of lactic acid with either or both of the salts may beused as the starting material, or a mixture of the salts.

While it is preferred to use apparatus of the type described generallyabove because of the advantages of continuously and readily controlledoperation, it will be understood that the mixture may be formed inseparate batches and these batches Supplied continuously to the ilashesterier. It will be understood, further, that other types of ashesterifying apparatus may be used than that described above, such as,for example, an esterifying apparatus utilizing a heated inert liquidinto which the mixture is sprayed or injected or a system in which themixture is superheated under increased pressure and then suddenlyrelieved of pressure to ash vaporize the volatiles and separate themfrom vthe solids. Moreover, the volatiles delivered from the flashesterier may be handled with dierent apparatus and in dilerent ways thanthat described above. For example, if desired, the volatiles from theilash esterier may be fractionally condensed in order to separate thealcohol vapor from the water and ester, leaving the mixture of thelatter two liquids for subsequent separation by azeotropic distillation.

In View of the many modifications possible in the process and in theapparatus, it will be understood that the above-described apparatus andtypical method described above are illustrative and should not beregarded as limiting the scope of the following claims.

We claim:

l. A method of preparing alkyl lactates comprising mixing an aliphaticalcohol, a strong mineral acid, and a member of the group consisting ofalkali metal lactates and alkaline earth metal lactates, suddenlysubjecting the mixture to such conditions of temperature and pressure asto ilash vaporize the volatile components of the mixture and separatethem from any solids present in the mixture.

2. A method of preparing alkyl lactates comprising mixing an aliphaticalcohol, a lactic acid salty of the group consisting of alkali .metallactates and alkaline earth metal lactates and a suicient amount of astrong mineral acid to provide a pH value between 0.7 and 1.4, raisingthe temperature of the mixture suddenly to a temperature suiicientlyhigh to produce a lactic acid ester of said alcohol and to flashvaporize the ester, the excess alcohol and water and separate them fromany solids present in the mixture.

3. A method of producing alkyl lactates comprising mixing a lactic acidsalt of the group consisting of alkali metal lactates and alkaline earthmetal lactates, an aliphatic alcohol and sufficient sulfuric acid toprovide a pH value between about 0.7 and 1.4, feeding said mixturecontinuously into a reaction chamber maintained at a temperaturesufciently high to produce a lactic acid ester of said alcohol and toflash vaporize said ester, said alcohol and any water present in saidsolution, and removing vapor and any solid components continuously andseparately from said reaction chamber.

4. A method of preparing methyl lactate comprising mixing about one partof lactate of the class consisting of alkali metal lactates and f 3alkaline earth metal lactates with about 3 to 15 parts of methyl alcoholin the presence of sufficient sulfuric acid to provide a pH valuebetween about 0.7 and 1.4 at a temperature slightly below the boilingpoint of methyl alcohol, increasing the temperature of the mixturesuddenly to produce methyl lactate and to flash vaporize the alcohol,said methyl lactate and any water present in the mixture, andwithdrawing the vapor to separate it from any solids present in themixture.

5. A method of preparing methyl lactate comprising mixing about one partof lactate of the class consisting of alkali metal lactates and alkalineearth metal lactates with about 3 to l5 parts of methyl alcohol in thepresence of sufllcient sulfuric acid `to provide a pH value betweenabout 0.7 and 1.4, Continuously preheating the mixture at a temperaturebelow the boiling point of methyl alcohol, increasing the temperature ofthe mixture suddenly to produce methyl lactate and to ash Vaporize thealcohol, the methyl lactate and any water present in the mixture, andwithdrawing the vapor to separate it from any solids present in themixture.

6. A method of preparing ethyl lactate comprising mixing about one partof lactate of the class consisting of alkali metal lactates and alkalineearth metal lactates with about 3 to 15 parts of ethyl alcohol in thepresence of sufficient sulfuric acid to provide a pH value between about0.7 and 1.4, heating the mixture at a temperature below the boilingpoint oi ethyl alcohol, increasing the temperature of the mixturesuddenly to produce ethyl lactate and to ilash vaporize vthe alcohol,said ethyl lactate and any water present in the mixture and withdrawingthe vapor to separate it from any solids present in the mixture.

7. A method of preparing ethyl lactate comprising mixing about one partof lactate of the class consisting of alkali metal lactates and alkalineearth metal lactates with about 3 to 15 parts of ethyl alcohol in thepresence of suicient sulfuric acid to provide a pH value between about0.7 and 1.4, continuously preheating the mixture at a temperature belowthe boiling point of ethyl alcohol, increasing the temperature of themixture suddenly to produce ethyl lactate and to liash vaporize thealcohol, said ethyl lactate and any water present in the mixture, andwithdrawing the vapor to separate it from any solids present in themixture.

8. A method of preparing alkyl lactates comprising mixing about one partof a member of the group consisting of alkali metal lactates andalkaline earth metal lactates with about 3 to 20 parts of an aliphaticalcohol in the presence of suicient strong mineral acid to provide a pHValue between 0.7 and 1.4, heating the mixture at a temperature belowthe boiling point of the alcohol, suddenly subjecting said mixture tosuch conditions of temperature and pressure as to produce an alkyllactate and to ilash vaporize the alcohol, alkyl lactate and any waterpresent in the mixture and withdrawing the vapor to separate it from anynon-volatile impurities present in the mixture.

SAMUEL M. wEisBERG. EDWIN G. sTiMPsoN. ALFRED L. MILLER.

